No matter what, if you are taking Oxycodone for the first time, no matter if it is instant or extended release, it is better to take dose that is too small than a dose that is too big. So I think that the recommendation of around 15 mg oxycodone is a sound recommendation. Some persons would feel euphoria and sedation from 5 mg oxycodone the first time, and if they took 20 or 30 mg they would get nausea and other side effects.
What I am saying is, that it is better to take to little than too much. So the way you took your first oxycodone was the correct method, even though you might feel you didn't get much effect. Next time you know that you can handle 15 mg, and maybe attempt 20-30 mg instead.
Opioid naive persons really have to be careful with oxycodone. It can be dangerous to take oxycodone as the first experience with opiats. Codeine might be a more secure first time opiat. But if you do start with oxycodone, you started in the correct way with a careful dose.
Instant release oxycodone will probably make a naive personer fell plenty at 10 mg, whereas extended release might need to be increased to 20 mg. Instant release is not really something I would advice opioid naive persons to use, as i can easy and fast result in nausea and often does the times I have seen a naive person take IR oxycodone. Extended release is more safe for naive persons in my experience. The dosage a naive person needs to take to "feel something" varies quite a lot and therefore, it is better to start with a small amount around 10-20 mg. Again, I have seen people puke from a 5 mg OxyContin while others can handle 20 mg. Therefore, the dose needed is depending on the individual which is why it is best to start with a small dose the first time. Better safe than sorry.